The overall objectives of this project are: 1) to study the mechanisms of transformation of normal mammary cells by the mouse mammary tumor virus (MTV); 2) to analyze the various steps of virus synthesis and release. Relationships between the cell membrane and virus envelopes will particularly retain attention; 3) to continue the search for a viral etiology for human breast cancer. To this effect a number of techniques developed in the past year will be used to increase our stock of human breast tumor cell lines. Detection of common surface antigens between mouse and human tumor cells will be investigated by immunofluorescence and radioimmune assays. Also, attempts will be made to determine whether a MuMTV related nucleic acid exist in human breast tumor cells.